Blog entry

2D indium selenide: novel semiconducting analogue of graphene

By Denis Bandurin,  U. Manchester.

The discovery of graphene, an atomically thin crystal of carbon atoms, has promised new technological breakthroughs and novel ultra-fast devices. One of the most exciting opportunities was the possibility to replace the conventional silicon industry with two-dimensional technology. However, after some time it became clear that graphene cannot compete with silicon despite having unique electronic properties. In order to understand this let us consider two physical properties of materials which allow transistor to operate and serve as building blocks in all modern devices.

The first property is called band gap. Band gap is a characteristic of any crystal. It accounts for the energy separation between allowed energy bands. By the size of this band gap all crystals can be divided into three main groups: metals (with zero gap), semiconductors (1-3 eV) and insulators (more than 3 eV). Semiconductors represent an important class of materials which form the basis of modern technology. Without going to much in detail let us remember that it is the band gap of semiconductors that allowed them to serve as building blocks for transistors. The second property is called electron mobility. This quantity is related to the motion of electrons when electric field is applied. The higher the mobility the faster the electrons can drift in electric field and the faster is the operation of transistors.

On the one hand, graphene has enormous room temperature mobility of electrons. This was one of the main reasons why it attracted so much attention as a material for transistor industry. However, graphene does not have a band gap. Hence the transistor made of it would not be able to compete with silicon technology.

That is why there has been an intensive search of new 2D materials which on the one hand would have distinct properties of graphene (such as high mobility of electrons, atomic thickness, stability under ambient conditions and so on) and on the other hand would represent a semiconductor family meaning that they would possess a finite band gap. So far only two decent 2D semiconductor competitors to graphene have emerged. These are few-layer dichalcogenides such as MoS2 and WSe2 and multilayer black phosphorous. 2D dichalcogenides have proven to be highly stable however the room temperature mobility of electrons in these materials are significantly lower than that of graphene and even silicon. On the other hand, 2D black phosphorous promises much higher mobility at room temperature but is so poorly stable under ambient conditions.

Recently another material, called two-dimensional Indium Selenide, has emerged as a potential candidate for semiconductor technology. Indium selenide belongs to the family of layered semiconductors. Each of its layers has a honeycomb lattice made of indium and selenium atoms. Layers are held together by relatively weak interactions that allows mechanical isolation of atomically thin films.

Even though InSe is relatively stable, atomically thin films suffered from degradation. In order to circumvent this problem InSe device were fabricated in an inert atmosphere of argon with subsequent encapsulation with hexagonal boron nitride (atomically flat insulator), hBN.

Atomically thin films of InSe inherited high room temperature electron mobility of charge carriers from its parent – bulk InSe. Thin InSe transistors operate faster than conventional silicon transistors. Also, InSe has a technologically favourable band gap (1.2 eV) which may find its application in ultrathin-body high mobility nanoelectronics. In addition, it turned out that the band gap of InSe changes drastically (around 2 times) when the thickness is changed from one to six layers. Thus it is possible to fabricate complex transistors with tuneable band gap just by stacking InSe layer by layer. 

At low-temperature InSe demonstrated even higher electron mobility that allowed the observation of the fully developed Quantum Hall Effect . It worth noting, that the latter has been found so far only in such 2D materials as graphene and black phosphorous. This makes InSe a very important material to study the low-dimensional phenomena.

To summarize, InSe is a novel two-dimensional semiconductor which offers a unique playground for development of fast ultrathin electronic and optoelectronic devices.